Which of the following are examples of the null set?
A = {x : x ϵ N, 1 < x ≤ 2}.
Natural numbers = 1, 2, 3, 4, 5, 6, 7,…
Natural number greater than 1 (1 < x) = 2, 3, 4, 5, ..
Natural number less than or equal to 2 (x ≥ 2) = 1
A number cannot be simultaneously greater than 1 and less than equal to 2
∴ no elements in this set
∴ It is a null set.